The nonlinear interactions of the instability modes in a hypersonic laminar
boundary layer undergoing natural transition are examined using bispectral
analysis. The data are from an experiment of a boundary layer flow on a co
oled-wall cone in a low-level free-stream disturbance hypersonic wind tunne
l, and thus, the bispectral measurements are a good representation of the n
atural transition processes. The bispectral analysis shows that in the init
ial stages of transition the dominant nonlinear interaction is forcing by t
he fundamental to generate a harmonic. Subsequently, mutual forcing of the
fundamental and harmonic yield a second harmonic. Difference interactions w
ithin the band of unstable disturbances centered on the fundamental and har
monic also generate a low frequency nonlinear interaction. At high amplitud
es of the fundamental and harmonic a nonlinear interaction characterized by
a low frequency modulation of the fundamental and harmonic then follows. T
his nonlinear interaction is then the most dominant and precedes the breakd
own of the laminar flow. (C) 1999 American Institute of Physics. [S1070-663
1(99)02312-0].